NR ANUZ
AU Pennisi,E.
TI Gene evolution. Cannibalism and prion disease may have been rampant in ancient humans.
QU Science 2003 Apr 11; 300(5617): 227-8
PT historical article; news
AB Epidemics of prion diseases - possibly spread by cannibalism - plagued prehistoric humans, a team proposes online in Science this week. Analysis of the worldwide distribution of the prion gene suggests that two gene variants have undergone "balancing selection" through exposure to toxic prions.
VT
Some call it the laughing disease; others, kuru. This neurodegenerative disorder is universally fatal and 40 years ago killed almost 10% of a small New Guinea tribe called the Fore. Now molecular biologists propose that similar epidemics plagued prehistoric humans. Both then and more recently, kuru, a prion disease, was transmitted through cannibalism, Simon Mead and John Collinge of University College London and their colleagues claim in a report online in Science this week (www.sciencemag.org/cgi/content/abstract/1083320). They base their conclusions on the worldwide distribution of variants of the prion gene.
The work lends support to the idea that ancient people once regularly munched on their peers. This conclusion will be controversial, says John Hardy, a geneticist at the National Institute on Aging in Bethesda, Maryland. Nonetheless, "I think [Collinge and colleagues] might be right."
Until 50 years ago, the Fore reportedly had a tradition of eating the dead. In the 1960s, Carleton Gajdusek of the National Institute of Neurological Diseases and Stroke in Bethesda demonstrated that kuru was an infectious disease: Once cannibalism was banned, kuru disappeared.
Gajdusek blamed a slow-growing virus for the disease, but now the prime suspect in kuru is a malformed miniature protein called a prion. Contorted prions cause other, native prions to misfold, clump together, and kill brain cells. A similar process is believed to cause Creutzfeldt-Jakob disease (CJD) in humans and scrapie in sheep. Although some prion diseases occur spontaneously, in many cases, humans or other animals contract them by eating infected tissue.
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Deadly epidemic. Prehistoric people may have succumbed to the
prion disease that killed this man from a New Guinea tribe.
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A decade ago, Collinge showed that people carrying two identical copies of the gene for the prion protein are more susceptible to developing CJD than people who carry two unmatched gene variants. Although the variants create proteins that differ by only one amino acid, the mismatch somehow protects people against the disease.
To understand the history of the prion gene, Collinge's team looked at DNA from the Fore and also from 1000 people representing other groups around the world. All ethnic groups examined carried two versions of the prion gene.
The variants' widespread existence suggests that they have been conserved throughout human history, the team claims. Based on additional comparisons across cultures and with chimp DNA, the group estimates that the variants arose 500,000 years ago. For most genes, one variant improves an organism's fitness more than the other; eventually the less beneficial one becomes rare or disappears. Because both prion variants are so prevalent, they "must have undergone strong natural selection pressure," Collinge claims.
Among the Fore, the number of women carrying one copy of each variant - socalled heterozygotes - instead of two copies of one or the other, was particularly high. His group found that 23 of the 30 women over age 50 - those old enough to have practiced cannibalism - were heterozygotes, whereas statistically only 15 were expected.
The high percentage of heterozygotes suggests that having one copy of each variant protected the Fore against kuru. Early research had pointed in this direction, but this is the first demonstration that the variants underwent "balancing selection," Collinge notes. Only a few genes are known to have been selected in this way; one is the hemoglobin gene. One version causes sickle cell anemia if someone inherits two copies, but the variant persists because it protects against malaria if paired with another variant.
The distribution of prion gene variants in human populations suggests that prion diseases were widespread early in human history. Less clear, however, is what made the diseases so common. There is a chance that epidemics of prion diseases arose from eating tainted meats. But the cannibal's diet may have been one of the most important sources of prion-tainted tissue. Kuru is the only prion disease known to reach epidemic proportions. Regardless of their source, "prion diseases may have ravaged human populations in the distant past," says Adriano Aguzzi of the University Hospital in Zürich.
William Arens, a social anthropologist at the State University of New York, Stony Brook, thinks Collinge and his colleagues are on the wrong track. Gajdusek never witnessed cannibalism, he insists. Nor has "cannibalism as a custom ever been seen," he points out. "I have a basic problem with [Collinge's] hypothesis."
But Tim White, an integrative biologist at the University of California, Berkeley, favors the cannibalism theory. "Their conclusion is consistent with findings from archaeology," he says. Evidence includes the discovery of bones with human teeth marks and fossilized human feces containing human proteins. White says the implications of the new line of evidence should be taken seriously: "Modern genetics have been structured by the past."
MH Animal; *Cannibalism; Creutzfeldt-Jakob Syndrome/genetics; Ethnic Groups; Evolution, Molecular; Genetic Predisposition to Disease; Heterozygote; History of Medicine, Ancient; Human; Kuru/epidemiology/history/transmission; New Guinea/epidemiology; Prion Diseases/epidemiology/genetics/*history/transmission; Prions/*genetics; *Selection (Genetics); Variation (Genetics)
SP englisch
PO USA